Add extended Oklab (if it becomes viable)

[facelessuser]

The idea would be to either implement extended Oklab as a separate color space or replace Oklab with this new extended implementation. It may be that in the beginning, Oklab extended should be a separate space.

Extended is not ready and more work would be needed, but if it became viable, this would be useful.

Okhsl and Oksv should continue to be implemented using the non-extended form as the compression/decompression of the LMS breaks the algorithm.

The extended form is not just an extended Oklab, it does alter values in existing gamuts, from sRGB all the way up.

w3c/csswg-drafts#9449 (comment)

If we truly want to move to OkLCh as a gamut mapping default, extended would be the way to go as it fixes the gamut mapping of extreme colors giving sane values for any color space.